Published online before print
September 27, 2007Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0706977104OPEN ACCESS ARTICLEGeophysics
Evidence for an extraterrestrial impact 12,900 years ago that contributed to the
megafaunal extinctions and the Younger Dryas cooling

Communicated by Steven M. Stanley, University of Hawaii at Manoa,
Honolulu, HI, July 26, 2007 (received for review March 13, 2007)

Acarbon-rich black layer, dating to
12.9 ka, has been previously
identified at 50 Clovis-age sites across North
America and appearscontemporaneous with the abrupt onset of Younger
Dryas (YD)cooling. The in situ bones of extinct Pleistocene
megafauna,along with Clovis tool assemblages, occur below this black
layerbut not within or above it. Causes for the extinctions, YD
cooling,and termination of Clovis culture have long been
controversial.In this paper, we provide evidence for an
extraterrestrial (ET)impact event at
12.9 ka, which we hypothesize caused abruptenvironmental changes that contributed to YD cooling, major
ecological reorganization, broad-scale extinctions, and rapidhuman
behavioral shifts at the end of the Clovis Period. Clovis-agesites
in North American are overlain by a thin, discrete layerwith varying
peak abundances of (i) magnetic grains with iridium,(ii)
magnetic microspherules, (iii) charcoal, (iv) soot, (v)carbon spherules, (vi) glass-like carbon containing nanodiamonds,and (vii) fullerenes with ET helium, all of which are evidencefor an ET impact and associated biomass burning at
12.9 ka.This layer also extends
throughout at least 15 Carolina Bays,which are unique, elliptical
depressions, oriented to the northwestacross the Atlantic Coastal
Plain. We propose that one or morelarge, low-density ET objects
exploded over northern North America,partially destabilizing the
Laurentide Ice Sheet and triggeringYD cooling. The shock wave,
thermal pulse, and event-relatedenvironmental effects (e.g.,
extensive biomass burning and foodlimitations) contributed to
end-Pleistocene megafaunal extinctionsand adaptive shifts among
PaleoAmericans in North America.

Fig. 5. The dark line shown above is the black mat (12.9 ka) along the
arroyo wall of the Murray Springs Clovis site in Arizona. The YDB markers,
including magnetic grains and microspherules, iridium, soot, and fullerenes with
ET helium, are present in the few centimeters just below the black mat at the
top of the underlying sediment. This lithologic break represents the surface at
the end of the Clovis period before the formation of the black mat. Clovis
artifacts, a fire pit, and an almost fully articulated skeleton of an adult
mammoth were recovered at Murray Springs with the black mat draped conformably
over them. Excavations by Vance Haynes, Jr., and colleagues also revealed
hundreds of mammoth footprints in the sand infilled by black mat sediments.
These footprints and the mammoth skeleton appear to have been preserved by rapid
burial after the YDB event (1). No in situ Clovis points and extinct
megafaunal remains have been recovered from in or above the black mat,
indicating that the mammoths (except in isolated cases) and Clovis hunting
technology disappeared simultaneously.

Fig. 6. Clovis and the Younger Dryas. Haynes, in Taylor et al.
(1), correlated the end of Clovis cultural adaptations with the onset of Younger
Dryas cooling and provided end-Clovis 14C dates that have been
calibrated to 12.92 ka for Murray Springs and 12.98 ka for Blackwater Draw, two
of the sites we analyzed. This graph displays a corresponding date of 12.9 ka
for the onset of the YD in Greenland GISP2 ice core data based on
paleotemperature analyses (ref. 2, in red) and changes in methane concentrations
(ref. 3, in blue). The onset of the YD was marked by a dramatic 8°C
drop in Greenland temperature in <150 years with an associated abrupt decrease
in atmospheric methane concentrations. We propose that these climatic changes
were triggered by the YD event at ≈12.9 ka.

Fig. 7. Aerial photo (U.S. Geological Survey) of a cluster of
elliptical and often overlapping Carolina Bays with raised rims in Bladen
County, North Carolina. The Bays have been contrast-enhanced and selectively
darkened for greater clarity. The largest Bays are several kilometers in length,
and the overlapping cluster of them in the center is ≈8 km long. Previous
researchers have proposed that the Bays are impact-related features.

Fig. 8. Lommel (1) is in northern Belgium, near the border with the
Netherlands. At 12.94 ka (2), this site was a large late Glacial sand ridge
covered by open forest at the northern edge of a marsh. More than 50
archaeological sites in this area indicate frequent visits by the late
Magdalenians, hunter-gatherers who were contemporaries of the Clovis culture in
North America. Throughout the Bölling-Allerod, eolian sediments known as the
Coversands blanketed the Lommel area. Then, just before the Younger Dryas began,
a thin layer of bleached sand was deposited and, in turn, was covered by the
dark layer marked "YDB" above. That stratum is called the Usselo Horizon and is
composed of fine to medium quartz sands rich in charcoal. The dark Usselo
Horizon is stratigraphically equivalent to the YDB layer and contains a similar
assemblage of impact markers (magnetic grains, magnetic microspherules, iridium,
charcoal, and glass-like carbon). The magnetic grains have a high concentration
of Ir (117 ppb), which is the highest value measured for all sites yet analyzed.
On the other hand, YDB bulk sediment analyses reveal Ir values below the
detection limit of 0.5 ppb, suggesting that the Ir carrier is in the magnetic
grain fraction. The abundant charcoal in this black layer suggests widespread
biomass burning. A similar layer of charcoal, found at many other sites in
Europe, including the Netherlands (3), Great Britain, France, Germany, Denmark,
and Poland (4), also dates to the onset of the Younger Dryas (12.9 ka) and,
hence, correlates with the YDB layer in North America. [Reproduced with
permission from Marc De Bie (Copyright 2004).]

Fig. 9. Research sites with calibrated YDB ages, including Lommel,
Belgium, shown in Inset. High-Ir sites are shown in green. For the Bays,
three of five sediment analyses revealed detectable Ir values, although
radiocarbon ages of the Bays are inconsistent. Sediments from sites with no
detectable Ir values (<0.5 ppb) are shown in brown. Sites with black mats are
marked with inverted triangles. The approximate extent of the North American ice
sheets at 12.9 ka is shown in blue-green, which is consistent with our
observations that all sites were ice-free at the time of the YD event.

Fig. 10. SEM photomicrographs of mostly individual particles of
submicrometer-sized soot (shown on filter paper at yellow arrows), measured at
1,969 ± 167 ppm from Blackville Bay T13 (Left),
and measured at 21 ± 7 ppm from Murray Springs (Right).
The soot levels and morphology from both sites are similar to those from the K/T
(1). Only two of eight sites tested exhibited soot, perhaps because of
unfavorable conditions for preservation at some sites. Soot was identified using
SEM imaging and quantified by particle size analysis and weighing (2).

Fig. 11. A 13C NMR spectrum of glass-like carbon from
Carolina Bay M33 in Myrtle Beach, South Carolina. This was produced on a Varian
Unity-200 NMR spectrometer operating at 50.2 MHz and equipped with a Doty
Scientific 7-mm Supersonic MAS probe. Spinning speeds of 6.5 kHz were used and a
variable-amplitude, cross-polarization pulse sequence was used with recycle
delays of 1 s and a contact time of 1 ms. The aliphatic carbon appears centered
at 38 ppm, which is typical of peaks representative of nanodiamonds, where small
diamond domains are formed in compressed aromatic/graphitic materials. Of the
≈9-10% aliphatic carbon, the inferred nanodiamond component is estimated to
represent ≈3% total carbon.

Fig. 12. Deposition rates [calculated by MG ´
D ´%A´p/100, where MG (measured in mg/g) is magnetic
grain concentration (Table 1), D (in cm) is the YDB layer (Table 1), %A
is percent mineral abundance (Table 2), and p
(in g/cm3) is the average mineral density.] for magnetic
microspherules, magnetic grains, and their principal components at YDB sites
ordered by distance from the Gainey, MI, site (upper scale). Microspherules,
magnetic grains, magnetite, silicates,, and water content all dramatically peak
at Gainey, suggesting that they are terrestrial products of a nearby impact.
Ilmenite/rutile concentrations peak at Topper and are higher than Gainey at all
sites, suggesting that they are high-velocity ejecta from an impact. Because
magnetic grains at Wally's Beach where recovered from inside an extinct
Pleistocene horse skull and may not be representative of the sediment, magnetic
grain concentrations there are normalized to those at the nearby Chobot site.

Fig. 13. Mammoth bone found with Clovis artifacts (from the Blackwater
Draw collection). This bone is stained yellow (arrow) and is highly radioactive
(3,000 ppm U) only on the upper side that was just below the black mat. Bones
found above or deeper below the black mat are neither stained nor highly
radioactive. INAA analysis determined a high U concentration (58 ppm) in YDB
sediment at Blackwater Draw, which is ≈10 times the concentration above or
below. High U content on fossil bones is due to well known diagenetic processes
(1) as confirmed by the corresponding low Th content (<1 ppm) on the stained
bone surface. During breakdown of organic material under anoxic conditions, bone
beds also may precipitate phosphatic minerals (2), which in turn scavenge and
concentrate U. If so, the U enrichment on the bones and in the YDB sediment may
have been enhanced by the abundance of bones and other Ca sources in the
extinction layer. High levels of radioactivity may, therefore, be potentially
useful as an additional diagnostic marker of the YDB layer.

Fig. 14. (Left) Radioactivity profiles measured with a Geiger
counter at Blackwater Draw and Murray Springs. (Right) Radioactivity in
bone fragments from Blackwater Draw sediments (1) are compared with U and Th
concentrations from Blackwater Draw sediment. Radioactivity peaks in both
sediment and bone fragments in the YDB due to high concentrations of U.

Fig. 15. Sediment concentrations for U, Th, Hf, Sc, and Sm peak in the
K/T boundary at Gubbio, Italy (A) (1), and the late Eocene Chesapeake Bay
impact (≈36 Ma) at Massignano, Italy (2), which produced one of the largest
known tektite strewnfields (B). (C and D) Radioactive
element concentrations also peak in the YDB at Blackwater Draw, NM, (C)
and Lake Hind, Manitoba, Canada (D). At Blackwater Draw, the uranium
increase as determined by INAA is especially large (58 ppm) and yielded the most
radioactive sediment analyzed in the study (SI Fig. 14). Concentrations (in ppm)
are shown on a log scale, and depth (in cm) is centered on the YDB layer. Ir,
Ni, and numerous other elements also peak at the YDB layer (presented in the
main text) and are considered to have resulted from impact processes.

Fig. 16. Zircon (ZrSiO4) is one of several heavy minerals
potentially enriched with U and Th that can be concentrated to form a
radioactive layer. (Left) We analyzed sediment samples at the Topper site
for Zr (red arrow), the major constituent of zircon, and found evidence for a
minor increase in zircon abundance in the YBD at Topper. When normalized to
crustal values, U (purple arrow), Th, and Hf concentrations changed in direct
relationship to the abundance of Zr, suggesting that zircon may account for some
of the increased radioactivity. (Right) In contrast, at Daisy Cave, U
decreased relative to zircon, indicating a negative correlation to sediment
radioactivity.

Fig. 17. Ti may appear in the heavy minerals ilmenite, rutile, and
titanite. (Left) At Topper, the presence of sedimentary Ti (red arrow)
correlates well with higher sedimentary levels of U (purple arrow) and Th. (Right)
However, at Daisy Cave, these relationships were negative, as with zircon. In
summary, heavy mineral concentrations tested do not correlate well with an
increase in sediment radioactivity at Daisy Cave but do so at Topper, where the
formation of lag deposits may have been influenced by the impact. Heavy minerals
may be concentrated through impact-related processes such as (i)
high-velocity winds associated with the shockwave; (ii) heavy rains and
flooding following the impact; and (iii) selective dissolution of
sediment by acidic conditions due to fallout and acid rain. However, it is
unlikely that lag deposits are typical of the YDB, because these sediment
sequences appear to be relatively continuous. Furthermore, such deposits would
have concentrated interplanetary dust particles (IDPs), and they would be
present in the magnetic fractions isolated from bulk sediments at BWD and Murray
Springs. However, these two sites do not show high 3He/4He
ratios in the magnetic fraction, such as would be present if the lag deposits
had concentrated the IDPs, nor does the He in the bulk sediment suggest any such
concentration at the boundary. Only the fullerenes concentrate ET He, which is
inconsistent with lag deposits and consistent with an impact event at the YDB.

SI Text

Research Sites. Murray Springs. Near Sierra Vista, AZ,
Murray Springs is one of several local Clovis mammoth kill-sites associated with
a chain of end-Pleistocene ponds at 12.9 ka. Sediments from the YDB layer are
mostly fine to coarse fluvial or lacustrine sand. A distinctive black mat, most
likely of algal origin, drapes conformably over the bones of butchered mammoths,
and a thin layer (<2 cm) that contains YDB markers lies at the base of the black
mat and immediately overlies the bones (1). The upper surfaces of some
Clovis-butchered mammoth bones, which were in direct contact with the YDB and
the black mat, exhibit slightly higher radioactivity and magnetic susceptibility
than the lower surfaces.

Blackwater Draw. Blackwater Draw, NM, is southwest of the town of
Clovis, which gave its name to the type of projectile points first found there.
It was a PaleoAmerican hunting site on the bank of a spring-fed waterhole, where
the black mat was found draped over bones of butchered mammoths and Clovis
artifacts. YDB markers are concentrated in a ≈2-cm layer of fine-grained fluvial
or lacustrine sediment that lies at the base of the black mat in the uppermost
stratigraphic horizon containing in situ mammal bones and Clovis
artifacts. The upper surfaces of some mammal bones were in direct contact with
the YDB or the black mat and exhibited very high levels of radioactivity. We
sampled a 2-m stratigraphic sequence spanning the YDB down into the deep gravels
that date to >40 ka and possibly to 1.6 Ma (2). ET markers peaked only in the
YDB.

Gainey. North of Detroit, MI, Gainey was a PaleoAmerican campsite
located tens of kilometers from the southern margin of the Laurentide Ice Sheet
at 12.9 ka. Sediments containing YDB markers are mostly fine alluvial sand and
glacial silt. The Gainey site has been closed and hence inaccessible for many
years, and only archived samples from the ≈5-cm YDB layer were available for
analysis. No black mat was observed.

Wally's Beach. At St. Mary Reservoir, southwestern AB, Canada,
Wally's Beach was a stream-fed valley that, at 12.9 ka, supported many species
of now-extinct megafauna, including mammoths, camels, and horses. Hundreds of
their footprints were found there during prior excavations. A sediment sample of
fine-grained and silty alluvium was provided to us by Brian Kooyman from the
brain cavity of a horse skull found in the YDB layer amidst Clovis points that
tested positive for horse protein, providing some of the first evidence that
Clovis peoples hunted horses (3).

Topper. Topper is located on a high bank of the Savannah River near
Allendale, SC, and was a Clovis-age flint quarry containing thousands of
artifacts. Sediments are eolian, fluvial, colluvial, and alluvial in origin and
are comprised mostly of coarse to medium quartz sand. YDB markers occur within a
≈5-cm interval immediately in and above a distinct layer of Clovis artifacts.
Lower sediments in the sequence have been dated to >55 ka (4), and no ET markers
appear in the stratigraphic sections above or below the YDB. There is no black
mat at this site.

At a new excavation, we used the neodymium magnet and a magnetic
susceptibility meter to help identify the YD layer based on the high iron
content. Shortly afterward, the excavators recovered part of a Clovis point
immediately beneath the YD layer, illustrating the usefulness of the YDB markers
for locating the Clovis horizon in new locations.

Chobot. Chobot is Southwest of Edmonton, AB, Canada. In Clovis times,
it was located along the shore of a proglacial lake, where a supply of quality
flint attracted hunter-gatherers. The presence of Clovis artifacts (5) dates
this level to an interval of ≈200 yr ending at 12,925 cal B.P. (6). The Clovis
level is capped by the YDB layer, above which there is a black mat similar to
other sites. The YDB sediment samples are mostly fine-grained and colluvial.

Daisy Cave. A cave/rockshelter on San Miguel Island, Daisy Cave is
one of the Channel Islands off the Southern California coast. This cave does not
appear to have been occupied until ≈11.5 ka, but a Clovis-age human skeleton was
found on nearby Santa Rosa Island, demonstrating that the PaleoAmericans had
boats capable of reaching the islands. Several markers were found, but others,
including Ir, were not found, possibly because the protected cave shelter
prevented accretion. The sediment with YDB markers dates to ≈13.09 ka (7) and
varies from fine sand to silt.

Lake Hind. In MN, Canada, Lake Hind was an end-Pleistocene proglacial
lake. Various analyses by Boyd et al. (8) show that prior to 12.76 ka,
the ice dams on the lake failed catastrophically as part of a regional pattern
of glacial lake drainages. In this study, we confirmed with calibrated
radiocarbon dating that the drainage took place at ≈12.76 ka (UCIAMS 29317). At
the YDB, the failure rapidly transformed the lake from deep to shallow water, as
shown by pollen analysis and the start of peat accumulation. The sample
sediments are fine-grained lacustrine silt and peat.

Morley. Morley is a nonarchaeological site west of Calgary in AB,
Canada. The site is on a raised drumlin, a subglacial erosional landform that
formed at the end of the Pleistocene during deglaciation (9). The largest
drumlin field near Ontario (5,000 km2) contains 3,000 drumlins that
date to shortly after 13 ka, and the age of the Morley drumlin field appears to
be similar. Later, the ice sheet melted away leaving atop the drumlin glacial
debris containing numerous YDB markers. Samples are mostly gravel grading down
through coarse and medium sand.

Lommel. Lommelis described in SI Fig. 8.

Carolina Bays. The Carolina Bays are a group of »500,000
highly elliptical and often overlapping depressions scattered throughout the
Atlantic Coastal Plain from New Jersey to Alabama (see SI Fig. 7). They range
from ≈50 m to ≈10 km in length (10) and are up to ≈15 m deep with their parallel
long axes oriented predominately to the northwest. The Bays have poorly
stratified, sandy, elevated rims (up to 7 m) that often are higher to the
southeast. All of the Bay rims examined were found to have, throughout their
entire 1.5- to 5-m sandy rims, a typical assemblage of YDB markers (magnetic
grains, magnetic microspherules, Ir, charcoal, soot, glass-like carbon,
nanodiamonds, carbon spherules, and fullerenes with 3He). In Howard
Bay, markers were concentrated throughout the rim, as well as in a discrete
layer (15 cm thick) located 4 m deep at the base of the basin fill and
containing peaks in magnetic microspherules and magnetic grains that are
enriched in Ir (15 ppb), along with peaks in charcoal, carbon spherules, and
glass-like carbon. In two Bay-lakes, Mattamuskeet and Phelps, glass-like carbon
and peaks in magnetic grains (16-17 g/kg) were found ≈4 m below the water
surface and 3 m deep in sediment that is younger than a marine shell hash that
dates to the ocean highstand of the previous interglacial.

Modern Fires. Four recent modern sites were surface-sampled. Two were
taken from forest underbrush fires in North Carolina that burned near Holly
Grove in 2006 and Ft. Bragg in 2007. Trees mainly were yellow pine mixed with
oak. There was no evidence of carbon spherules and only limited evidence of
glass-like carbon, which usually was fused onto much larger pieces of charcoal.
The glass-like carbon did not form on oak charcoal, being visible only on pine
charcoal, where it appears to have formed by combustion of highly flammable pine
resin.

Two surface samples also were taken from recent modern fires in Arizona; they
were the Walker fire, which was a forest underbrush fire in 2007 and the Indian
Creek Fire near Prescott in 2002, which was an intense crown fire. Trees mainly
were Ponderosa pine and other species of yellow pine. Only the crown fire
produced carbon spherules, which were abundant (≈200 per kg of surface sediment)
and appeared indistinguishable from those at Clovis sample sites. Both sites
produced glass-like carbon fused onto pine charcoal.

Methods. Separation of the magnetic fraction from sediment.
Initially, we used the magnet for in situ field testing to help locate
the peak in grains in the YDB. However, such testing works only under the most
favorable conditions, such as in loose, dry sediment with a high concentration
of grains. If the samples contain high percentages of clay or are damp, we found
that the magnet performs poorly. In addition, even if conditions are ideal but
the concentration of grains is low, such as <1g/kg, we found it difficult to
quantify the amount of grains on the magnet in the field. In summary, we found
it far simpler to locate the YDB by analyzing magnetic grain abundances in the
laboratory following the procedures below. We used only grade-42 or higher
neodymium magnets, having found that nearly all other magnets are too weak and
that some will completely fail to extract any magnetic grains. Typically, we
used the size 2 ´1 ´ 0.5
inches (1 inch = 2.54 cm), which was convenient for field and laboratory work.

Although sonication is a common way to separate magnetic grains, the process
was not used in our studies, because the procedure typically collects only the
smallest and most magnetic grains, excluding up to 90% of the remainder,
including many of the most interesting items, such as titanium-rich
microspherules.

Typically, several methods were used to separate magnetic grains from
sediment, depending upon the type of sediment. For large-scale processing, the
following basic procedures were used with automated equipment and a bank of
magnets, which were placed in a moving stream of either wet or dry sediment.
Small samples were processed manually.

Loose or sandy sediment. About 500-1,000 g of friable sand or silt was
first dehydrated at room temperature and weighed, and then, the samples were put
into a container and the lumps were broken up. All of the processing was done
using non-metallic tools to avoid adding foreign metal to the sample, and care
was taken not to crush the fragile carbon component, if it was to be extracted
also. Next, the magnet was placed in a 4-mil plastic bag to prevent grains from
sticking directly to the magnet. A sediment sample was poured over the tightly
bagged magnet into an empty container. Magnetic grains stuck to the magnet, and
when the magnet was removed from the bag, the grains fell into a separate
container. The process was repeated until nearly all of the grains were
recovered.

Final step. As an essential final step to remove dust and debris,
which can conceal the magnetic grains and spherules, the magnetic fraction was
placed in a beaker of water. Then, the bagged magnet was gently agitated in the
beaker to attract the magnetic grains. These were then deposited on a dry lab
dish, by touching the wet bag to the plate after the magnet was removed from the
bag. After drying, the sample was weighed, catalogued, and examined
microscopically at ´100-150 magnification.

Sticky or clayey sediment. For sediment that was difficult to pulverize,
we added ≈4 liters of water to each 500-1,000 g of sediment and homogenized it
into slurry. The bagged magnet was then used to extract magnetic grains from the
fluidized mixture. The magnetic grains were then released from the magnet into a
separate container of water and then retrieved onto a laboratory dish as in the
final step discussed above.

Extraction of magnetic microspherules. To find microspherules, the
magnetic fraction was extracted from a weighed sediment sample with the
neodymium magnet. We found it essential to complete the final step of cleaning
the magnetic fraction with water, as outlined above. Also, because there are
relatively few microspherules in bulk sediment, it was often necessary to
inspect the most or all of the magnetic fraction extracted from 500-1,000 g of
sediment. Next, one or more ≈100 mg aliquots of the magnetic fraction were
weighed, dusted sparsely across a microscope slide, and scanned microscopically.
Microspherules, which typically ranged from 10-100 mm,
were counted, and abundances were extrapolated to quantity per kg. While viewed
at ´100-150 magnification, selected microspherules
were removed from the magnetic fraction manually with a moistened probe and
placed onto an SEM mount or double-sided tape on a microscope slide. These
spherules were either left whole or sectioned and given a microprobe polish for
analysis by laser ablation or x-ray fluorescence (SEM/XRF).

Extraction of carbon spherules, glass-like carbon, and charcoal.
Carbon spherules have a low specific gravity, and water floatation was used to
assist with their separation. Typically, one kg of sample was added to ≈4 liters
of water and agitated. The floating fraction was captured with a 150-mm
sieve. In addition, there was often a carbon fraction with a specific gravity
slightly higher than that of water, and that was removed from the top of the wet
sediment visually. After drying at low temperatures, the carbon spherules were
collected either visually or gravimetrically by vibrating the dried sample on an
inclined, polished surface. Glass-like carbon and charcoal, contained in the
same sample, were extracted manually and weighed.

Radiocarbon. AMS radiocarbon dating was performed by J. Southon (Keck
Carbon Cycle AMS Facility) on peat and silt from Lake Hind. The radiocarbon date
was converted to calibrated dates using IntCal04 (11).

Inductively coupled plasma mass spectrometry (ICP-MS) analysis. ICP-MS
analyses. The isotopes evaluated for this investigation were: 52,53Cr
; 58,60,61,62,64Ni ; and 191,193Ir . Uncertainties varied
by isotope, but all were less than ±20%. These
isotopes were selected to evaluate the possibility of ET material in the
sediment samples. Only Ir showed anomalous values. More details on the rationale
for the selection of these isotopes, the ICP-MS conditions, analytical details
of other isotopes not reported here, and the results and basis of the elements
selected for further study will be presented in a forthcoming paper. This suite
of isotopes allowed the use of aqua regia type acid mixtures to facilitate
digestions. The digestion scheme allowed elements on the outside versus inside
of the particles to be studied separately.

The analysis process involved digestion with concentrated Fisher OPTIMA
nitric acid (HNO3) and then concentrated Fisher OPTIMA hydrofluoric
acid (HF) with evaporation of the hydrofluoric acid before ICP-MS analysis in 5%
(vol/vol) HNO3. All vessels and containers were acid washed in 10%
nitric acid overnight, rinsed with ASTM I water, and dried beforehand.

Digestions. Initially, large sample weights of ≈100 g were used to screen
the various isotope ratio changes to detect changes in uranium (U) isotopes. A
method blank and a positive control (National Institutes of Standards and
Technology, Buffalo River Sediment SRM 8704) were analyzed in parallel.

Screening digestions. Each 100-g sample ground in a mortar and pestle to
pass through a 149-mm sieve was allowed to digest
overnight in 75 ml of concentrated nitric acid in a Teflon beaker of known
weight in a fume hood. The temperature on a hot plate was stepped-through for 2
h with Teflon watch glasses on at 50-55°C, 70-75°C,
80-85°C, 90-95°C, 100-105°C,
and 110-115°C and then allowed to reflux with Teflon
watch glasses until there were no more brown fumes. The gradual ramp was
necessary to avoid boil-over and bumping of the heterogeneous digestion mixture.
This took up to a week. After cooling, 75 ml of concentrated HF was added and
allowed to stop bubbling. After 2 h, the above temperature ramp was repeated
with watch glasses on after adding another 45 ml HF. The watch glasses were
removed and the 2 h temperature steps then done at 125-130°C,
135-140°C, 145-150°C,
165-170° C, 175-180°C, and
195-200°C until dry. After cooling, the residue was
weighed, and broken up with an acid washed pestle/Teflon spatula. The same
process was repeated on the residue with 60 ml of concentrated HF alone, and
then another 60 ml.

The dried broken up digestion residue (usually between 36% and 78% of the
original weight) was extracted with 5% nitric acid in 2-h steps at 50°C,
with each liquid being combined by decantation after cooling in a small 150-ml
Teflon beaker where the combined dilute nitric acids extractions were evaporated
at 110-115°C. This amalgamate was evaporated to ≈15
ml and used for analysis after cooling. This solution had a precipitate after
standing and the liquid portion was carefully decanted into another centrifuge
tube that was then centrifuged at 900 ´g. The
solids of the dilute HNO3 extraction steps and the centrifugation
step were combined, dried to constant weight, and set aside for further analysis
by nondestructive analytical chemistry techniques. This residual material was
≈27-64% of the original weight. The centrifuge tube supernatant was analyzed by
ICP-MS, and its dry weight was ≈3.5-5.6% of the original weight. The solutions
were yellow, orange, or red to orange compared to colorless for the method
blank, and orange for the NIST sediment sample.

After the initial screening results were analyzed, small amounts of samples
of 1 g were then digested to provide a HNO3/HCl available fraction, a
HF available fraction after nitric acid digestion, and a residual fraction.

Small-weight digestions. Approximately 1 g of the sample ground in a
mortar and pestle to pass through a 149-mm sieve was
digested twice in a Teflon beaker with concentrated HNO3 (30 ml at
room temperature for 16 h, 55-60°C for 16 h, 85-90°C
for 16 h, then 135-140°C for 16 h before cooling and
decanting the supernatant into another Teflon beaker, followed by 20 ml of
concentrated nitric acid digestion for 16 h at 135-140°C)
followed by three digestions with 4:1 HCl/HNO3 (20 ml
´ 3, each for 16 h at 135-140°C).
Each residue with solids was dried at 135-140°C
before the next digestion (SR1). The final solid residue after the last 4:1
HCl/HNO3 leaching was also retained. The 5 extractions were combined,
evaporated, and the dry broken up residue leached with 30 ml 5% (vol/vol) nitric
acid at 100°C to constitute the soluble phase that
was centrifuged for 10 min at 900 ´g. The
solid residues (from leaching with 5% nitric acid and centrifugation) were
combined with SR1 for the HF digestions

Two HF digestions followed, one with 4:1 HF/HNO3 (one with 30 ml
for 16 h at room temperature, 55-60°C for 16 h, and
then 135-140°C for 16 h) and decanting the
supernatant into another Teflon beaker. The second digestion of the dried
residue with particulates was with 30 ml of concentrated HF, and the second
leaching was combined with the first extraction, dried, and the solid then
leached with 5% nitric acid, and the leachate centrifuged. The centrifugation
solids and leachate solids were combined with the solid from the second HF
digestion step and then dried. The Teflon beakers were then scraped with a
Teflon spatula to provide the residual solid weight that varied from 0.02% to
10% of the original weight. These residues were further analyzed by
nondestructive instrumental analysis. The colors of these residual solids varied
from black/greasy and black/hard to white flake (samples) to yellow-orange/cream
(method blank and the National Institute of Standards and Technology sample).
The digestion of small samples was thus at least 90% efficient in digesting the
original sample.

X-ray Fluorescence (XRF). Representative microspherules were sliced,
polished, and mounted for analysis by XRF with a scanning electron microscope
(SEM) by B. Cannon (Cannon Microprobe). The x-ray spectra were obtained using an
ARL SEMQ Electron Microprobe operated at 20 kV accelerating voltage, 50-nA beam
current, 52.5° x-ray take off angle, a Kevex 2003
energy dispersive x-ray detector (EDS) biased at 620 V with an 8-mm-thick
beryllium window and a PGT MCA 4000 multichannel analyzer. The resolution
of the detector is 159 eV at Mn K alpha. Elements with atomic number 10 and
smaller are not detected by this system due in part to thickness of the
beryllium window on the detector. Different regions of the microspherules were
randomly analyzed to obtain average elemental concentrations.

Prompt gamma-ray activation analysis (PGAA). PGAA of samples from
many sites was performed at the Department of Nuclear Research, Institute of
Isotopes in Hungary. PGAA is a non-destructive technique (12), using neutron
beams to excite the samples producing gamma-ray spectra unique to each element.
Typically, several gamma-rays are excited for each element, which can be used
for analysis. PGAA is sensitive to the main constituents, except oxygen, and
many trace elements in a sample. Concentrations are normalized to the total
sample composition assuming standard oxidation states. Bulk samples of magnetic
grains and microspherules, ranging in size from 9 mg to 13 g were analyzed with
PGAA for H, B, F, Na, Al, S, Si, Mg, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, Cd, Sm, Eu, and Gd. Uncertainties varied by element, but all were less than
±20%.

Nneutron activation analysis (NAA). The analysis of samples from many
sites was performed at Becquerel and Activation Laboratories in Canada and at
the Department of Nuclear Research, Institute of Isotopes, in Hungary. NAA was
used to analyze trace element concentrations in both bulk sediment and magnetic
grain samples, which were analyzed for Na, Si, Ca, Sc, Cr, Fe, Co, Zn, As, Se,
Br, Rb, Zr, Mo, Ag, Cd, Sn, Sb, Te, Cs, Ba, Ce, La, Nd, Sm, Eu, Tb, Yb, Lu, Hf,
Ta, W, Ir, Au, Hg, Th, and U. Uncertainties varied by element, but all were less
than ±20%.